Re: [PATCH v1] proc/vmcore: fix false positive lockdep warning

From: Boqun Feng
Date: Wed Jan 19 2022 - 10:09:15 EST

Hi,

On Wed, Jan 19, 2022 at 12:37:02PM +0100, David Hildenbrand wrote:
> Lockdep complains that we do during mmap of the vmcore:
> down_write(mmap_lock);
> down_read(vmcore_cb_rwsem);
> And during read of the vmcore:
> down_read(vmcore_cb_rwsem);
> down_read(mmap_lock);
>
> We cannot possibly deadlock when only taking vmcore_cb_rwsem in read
> mode, however, it's hard to teach that to lockdep.
>

Lockdep warned about the above sequences because rw_semaphore is a fair
read-write lock, and the following can cause a deadlock:

TASK 1 TASK 2 TASK 3
====== ====== ======
down_write(mmap_lock);
down_read(vmcore_cb_rwsem)
down_write(vmcore_cb_rwsem); // blocked
down_read(vmcore_cb_rwsem); // cannot get the lock because of the fairness
down_read(mmap_lock); // blocked

IOW, a reader can block another read if there is a writer queued by the
second reader and the lock is fair.

So there is a deadlock possiblity.

Regards,
Boqun


> So instead, convert to srcu to make lockdep happy. We need srcu as our
> callbacks can sleep. Witht his change, I cannot trigger any lockdep
> complaint.
>
> [ 6.386519] ======================================================
> [ 6.387203] WARNING: possible circular locking dependency detected
> [ 6.387965] 5.17.0-0.rc0.20220117git0c947b893d69.68.test.fc36.x86_64 #1 Not tainted
> [ 6.388899] ------------------------------------------------------
> [ 6.389657] makedumpfile/542 is trying to acquire lock:
> [ 6.390308] ffffffff832d2eb8 (vmcore_cb_rwsem){.+.+}-{3:3}, at: mmap_vmcore+0x340/0x580
> [ 6.391290]
> [ 6.391290] but task is already holding lock:
> [ 6.391978] ffff8880af226438 (&mm->mmap_lock#2){++++}-{3:3}, at: vm_mmap_pgoff+0x84/0x150
> [ 6.392898]
> [ 6.392898] which lock already depends on the new lock.
> [ 6.392898]
> [ 6.393866]
> [ 6.393866] the existing dependency chain (in reverse order) is:
> [ 6.394762]
> [ 6.394762] -> #1 (&mm->mmap_lock#2){++++}-{3:3}:
> [ 6.395530] lock_acquire+0xc3/0x1a0
> [ 6.396047] __might_fault+0x4e/0x70
> [ 6.396562] _copy_to_user+0x1f/0x90
> [ 6.397093] __copy_oldmem_page+0x72/0xc0
> [ 6.397663] read_from_oldmem+0x77/0x1e0
> [ 6.398229] read_vmcore+0x2c2/0x310
> [ 6.398742] proc_reg_read+0x47/0xa0
> [ 6.399265] vfs_read+0x101/0x340
> [ 6.399751] __x64_sys_pread64+0x5d/0xa0
> [ 6.400314] do_syscall_64+0x43/0x90
> [ 6.400778] entry_SYSCALL_64_after_hwframe+0x44/0xae
> [ 6.401390]
> [ 6.401390] -> #0 (vmcore_cb_rwsem){.+.+}-{3:3}:
> [ 6.402063] validate_chain+0x9f4/0x2670
> [ 6.402560] __lock_acquire+0x8f7/0xbc0
> [ 6.403054] lock_acquire+0xc3/0x1a0
> [ 6.403509] down_read+0x4a/0x140
> [ 6.403948] mmap_vmcore+0x340/0x580
> [ 6.404403] proc_reg_mmap+0x3e/0x90
> [ 6.404866] mmap_region+0x504/0x880
> [ 6.405322] do_mmap+0x38a/0x520
> [ 6.405744] vm_mmap_pgoff+0xc1/0x150
> [ 6.406258] ksys_mmap_pgoff+0x178/0x200
> [ 6.406823] do_syscall_64+0x43/0x90
> [ 6.407339] entry_SYSCALL_64_after_hwframe+0x44/0xae
> [ 6.407975]
> [ 6.407975] other info that might help us debug this:
> [ 6.407975]
> [ 6.408945] Possible unsafe locking scenario:
> [ 6.408945]
> [ 6.409684] CPU0 CPU1
> [ 6.410196] ---- ----
> [ 6.410703] lock(&mm->mmap_lock#2);
> [ 6.411121] lock(vmcore_cb_rwsem);
> [ 6.411792] lock(&mm->mmap_lock#2);
> [ 6.412465] lock(vmcore_cb_rwsem);
> [ 6.412873]
> [ 6.412873] *** DEADLOCK ***
> [ 6.412873]
> [ 6.413522] 1 lock held by makedumpfile/542:
> [ 6.414006] #0: ffff8880af226438 (&mm->mmap_lock#2){++++}-{3:3}, at: vm_mmap_pgoff+0x84/0x150
> [ 6.414944]
> [ 6.414944] stack backtrace:
> [ 6.415432] CPU: 0 PID: 542 Comm: makedumpfile Not tainted 5.17.0-0.rc0.20220117git0c947b893d69.68.test.fc36.x86_64 #1
> [ 6.416581] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
> [ 6.417272] Call Trace:
> [ 6.417593] <TASK>
> [ 6.417882] dump_stack_lvl+0x5d/0x78
> [ 6.418346] print_circular_bug+0x5d7/0x5f0
> [ 6.418821] ? stack_trace_save+0x3a/0x50
> [ 6.419273] ? save_trace+0x3d/0x330
> [ 6.419681] check_noncircular+0xd1/0xe0
> [ 6.420217] validate_chain+0x9f4/0x2670
> [ 6.420715] ? __lock_acquire+0x8f7/0xbc0
> [ 6.421234] ? __lock_acquire+0x8f7/0xbc0
> [ 6.421685] __lock_acquire+0x8f7/0xbc0
> [ 6.422127] lock_acquire+0xc3/0x1a0
> [ 6.422535] ? mmap_vmcore+0x340/0x580
> [ 6.422965] ? lock_is_held_type+0xe2/0x140
> [ 6.423432] ? mmap_vmcore+0x340/0x580
> [ 6.423893] down_read+0x4a/0x140
> [ 6.424321] ? mmap_vmcore+0x340/0x580
> [ 6.424800] mmap_vmcore+0x340/0x580
> [ 6.425237] ? vm_area_alloc+0x1c/0x60
> [ 6.425661] ? trace_kmem_cache_alloc+0x30/0xe0
> [ 6.426174] ? kmem_cache_alloc+0x1e0/0x2f0
> [ 6.426641] proc_reg_mmap+0x3e/0x90
> [ 6.427052] mmap_region+0x504/0x880
> [ 6.427462] do_mmap+0x38a/0x520
> [ 6.427842] vm_mmap_pgoff+0xc1/0x150
> [ 6.428260] ksys_mmap_pgoff+0x178/0x200
> [ 6.428701] do_syscall_64+0x43/0x90
> [ 6.429126] entry_SYSCALL_64_after_hwframe+0x44/0xae
> [ 6.429745] RIP: 0033:0x7fc7359b8fc7
> [ 6.430157] Code: 00 00 00 89 ef e8 69 b3 ff ff eb e4 e8 c2 64 01 00 66 90 f3 0f 1e fa 41 89 ca 41 f7 c1 ff 0f 00 00 75 10 b8 09 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 21 c3 48 8b 05 21 7e 0e 00 64 c7 00 16 00 00
> [ 6.432147] RSP: 002b:00007fff35b4c208 EFLAGS: 00000246 ORIG_RAX: 0000000000000009
> [ 6.432970] RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00007fc7359b8fc7
> [ 6.433746] RDX: 0000000000000001 RSI: 0000000000400000 RDI: 0000000000000000
> [ 6.434529] RBP: 000055a1125ecf10 R08: 0000000000000003 R09: 0000000000002000
> [ 6.435310] R10: 0000000000000002 R11: 0000000000000246 R12: 0000000000002000
> [ 6.436093] R13: 0000000000400000 R14: 000055a1124269e2 R15: 0000000000000000
> [ 6.436887] </TASK>
>
> Reported-by: Baoquan He <bhe@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Baoquan He <bhe@xxxxxxxxxx>
> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
> Cc: Dave Young <dyoung@xxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>
> Based on next-20220118
>
> ---
> fs/proc/vmcore.c | 41 ++++++++++++++++++++++-------------------
> 1 file changed, 22 insertions(+), 19 deletions(-)
>
> diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
> index 702754dd1daf..edeb01dfe05d 100644
> --- a/fs/proc/vmcore.c
> +++ b/fs/proc/vmcore.c
> @@ -62,7 +62,8 @@ core_param(novmcoredd, vmcoredd_disabled, bool, 0);
> /* Device Dump Size */
> static size_t vmcoredd_orig_sz;
>
> -static DECLARE_RWSEM(vmcore_cb_rwsem);
> +static DEFINE_SPINLOCK(vmcore_cb_lock);
> +DEFINE_STATIC_SRCU(vmcore_cb_srcu);
> /* List of registered vmcore callbacks. */
> static LIST_HEAD(vmcore_cb_list);
> /* Whether the vmcore has been opened once. */
> @@ -70,8 +71,8 @@ static bool vmcore_opened;
>
> void register_vmcore_cb(struct vmcore_cb *cb)
> {
> - down_write(&vmcore_cb_rwsem);
> INIT_LIST_HEAD(&cb->next);
> + spin_lock(&vmcore_cb_lock);
> list_add_tail(&cb->next, &vmcore_cb_list);
> /*
> * Registering a vmcore callback after the vmcore was opened is
> @@ -79,14 +80,14 @@ void register_vmcore_cb(struct vmcore_cb *cb)
> */
> if (vmcore_opened)
> pr_warn_once("Unexpected vmcore callback registration\n");
> - up_write(&vmcore_cb_rwsem);
> + spin_unlock(&vmcore_cb_lock);
> }
> EXPORT_SYMBOL_GPL(register_vmcore_cb);
>
> void unregister_vmcore_cb(struct vmcore_cb *cb)
> {
> - down_write(&vmcore_cb_rwsem);
> - list_del(&cb->next);
> + spin_lock(&vmcore_cb_lock);
> + list_del_rcu(&cb->next);
> /*
> * Unregistering a vmcore callback after the vmcore was opened is
> * very unusual (e.g., forced driver removal), but we cannot stop
> @@ -94,7 +95,9 @@ void unregister_vmcore_cb(struct vmcore_cb *cb)
> */
> if (vmcore_opened)
> pr_warn_once("Unexpected vmcore callback unregistration\n");
> - up_write(&vmcore_cb_rwsem);
> + spin_unlock(&vmcore_cb_lock);
> +
> + synchronize_srcu(&vmcore_cb_srcu);
> }
> EXPORT_SYMBOL_GPL(unregister_vmcore_cb);
>
> @@ -103,9 +106,8 @@ static bool pfn_is_ram(unsigned long pfn)
> struct vmcore_cb *cb;
> bool ret = true;
>
> - lockdep_assert_held_read(&vmcore_cb_rwsem);
> -
> - list_for_each_entry(cb, &vmcore_cb_list, next) {
> + list_for_each_entry_srcu(cb, &vmcore_cb_list, next,
> + srcu_read_lock_held(&vmcore_cb_srcu)) {
> if (unlikely(!cb->pfn_is_ram))
> continue;
> ret = cb->pfn_is_ram(cb, pfn);
> @@ -118,9 +120,9 @@ static bool pfn_is_ram(unsigned long pfn)
>
> static int open_vmcore(struct inode *inode, struct file *file)
> {
> - down_read(&vmcore_cb_rwsem);
> + spin_lock(&vmcore_cb_lock);
> vmcore_opened = true;
> - up_read(&vmcore_cb_rwsem);
> + spin_unlock(&vmcore_cb_lock);
>
> return 0;
> }
> @@ -133,6 +135,7 @@ ssize_t read_from_oldmem(char *buf, size_t count,
> unsigned long pfn, offset;
> size_t nr_bytes;
> ssize_t read = 0, tmp;
> + int idx;
>
> if (!count)
> return 0;
> @@ -140,7 +143,7 @@ ssize_t read_from_oldmem(char *buf, size_t count,
> offset = (unsigned long)(*ppos % PAGE_SIZE);
> pfn = (unsigned long)(*ppos / PAGE_SIZE);
>
> - down_read(&vmcore_cb_rwsem);
> + idx = srcu_read_lock(&vmcore_cb_srcu);
> do {
> if (count > (PAGE_SIZE - offset))
> nr_bytes = PAGE_SIZE - offset;
> @@ -165,7 +168,7 @@ ssize_t read_from_oldmem(char *buf, size_t count,
> offset, userbuf);
> }
> if (tmp < 0) {
> - up_read(&vmcore_cb_rwsem);
> + srcu_read_unlock(&vmcore_cb_srcu, idx);
> return tmp;
> }
>
> @@ -176,8 +179,8 @@ ssize_t read_from_oldmem(char *buf, size_t count,
> ++pfn;
> offset = 0;
> } while (count);
> + srcu_read_unlock(&vmcore_cb_srcu, idx);
>
> - up_read(&vmcore_cb_rwsem);
> return read;
> }
>
> @@ -568,18 +571,18 @@ static int vmcore_remap_oldmem_pfn(struct vm_area_struct *vma,
> unsigned long from, unsigned long pfn,
> unsigned long size, pgprot_t prot)
> {
> - int ret;
> + int ret, idx;
>
> /*
> - * Check if oldmem_pfn_is_ram was registered to avoid
> - * looping over all pages without a reason.
> + * Check if a callback was registered to avoid looping over all
> + * pages without a reason.
> */
> - down_read(&vmcore_cb_rwsem);
> + idx = srcu_read_lock(&vmcore_cb_srcu);
> if (!list_empty(&vmcore_cb_list))
> ret = remap_oldmem_pfn_checked(vma, from, pfn, size, prot);
> else
> ret = remap_oldmem_pfn_range(vma, from, pfn, size, prot);
> - up_read(&vmcore_cb_rwsem);
> + srcu_read_unlock(&vmcore_cb_srcu, idx);
> return ret;
> }
>
> --
> 2.34.1
>