[RFC PATCH 0/2] ipc: do not hold ipc lock more than necessary

[Davidlohr Bueso]

The following set of not-thoroughly-tested patches are based on the
discussion of holding the ipc lock unnecessarily, such as for permissions
and security checks:

https://lkml.org/lkml/2013/2/28/540

Patch 0/1: Introduces new functions, analogous to ipc_lock and ipc_lock_check
in the ipc utility code, allowing to obtain the ipc object without holding the lock.

Patch 0/2: Use the new functions and only acquire the ipc lock when needed.

With Rik's semop-multi.c microbenchmark we can see the following
results:

256 sems without patches:
+  59.40%            a.out  [kernel.kallsyms]  [k] _raw_spin_lock
+   6.14%            a.out  [kernel.kallsyms]  [k] sys_semtimedop
+   3.84%            a.out  [kernel.kallsyms]  [k] avc_has_perm_flags
+   3.64%            a.out  [kernel.kallsyms]  [k] __audit_syscall_exit
+   2.06%            a.out  [kernel.kallsyms]  [k] copy_user_enhanced_fast_string
+   1.86%            a.out  [kernel.kallsyms]  [k] ipc_lock
+   1.75%            a.out  [kernel.kallsyms]  [k] __audit_syscall_entry
+   1.69%            a.out  [kernel.kallsyms]  [k] ipc_has_perm.isra.21
+   1.47%            a.out  [kernel.kallsyms]  [k] do_smart_update
+   1.43%            a.out  [kernel.kallsyms]  [k] pid_vnr
+   1.39%            a.out  [kernel.kallsyms]  [k] try_atomic_semop.isra.5

total operations: 151452270, ops/sec 5048409

256 sems with patches:
+  17.47%            a.out  [kernel.kallsyms]  [k] _raw_spin_lock
+  11.08%            a.out  [kernel.kallsyms]  [k] sys_semtimedop
+   8.81%            a.out  [kernel.kallsyms]  [k] avc_has_perm_flags
+   7.96%            a.out  [kernel.kallsyms]  [k] ipc_has_perm.isra.21
+   6.50%            a.out  [kernel.kallsyms]  [k] __audit_syscall_exit
+   4.67%            a.out  [kernel.kallsyms]  [k] ipc_obtain_object_check
+   4.19%            a.out  [kernel.kallsyms]  [k] ipcperms
+   3.75%            a.out  [kernel.kallsyms]  [k] copy_user_enhanced_fast_string
+   3.38%            a.out  [kernel.kallsyms]  [k] system_call
+   3.05%            a.out  [kernel.kallsyms]  [k] try_atomic_semop.isra.5
+   2.70%            a.out  [kernel.kallsyms]  [k] do_smart_update
+   2.60%            a.out  [kernel.kallsyms]  [k] __audit_syscall_entry

total operations: 266502912, ops/sec 8883430

While the _raw_spin_lock time is drastically reduced, others do increase.
This results in an overall speedup of ~1.7x regarding ops/sec.


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