Re: [PATCH -v4] use per cpu data for single cpu ipi calls

[Ingo Molnar]

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> Author: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Date:   Thu Jan 29 10:08:01 2009 -0500
> 
>     generic-ipi: use per cpu data for single cpu ipi calls
> 
>     The smp_call_function can be passed a wait parameter telling it to
>     wait for all the functions running on other CPUs to complete before
>     returning, or to return without waiting. Unfortunately, this is
>     currently just a suggestion and not manditory. That is, the
>     smp_call_function can decide not to return and wait instead.
> 
>     The reason for this is because it uses kmalloc to allocate storage
>     to send to the called CPU and that CPU will free it when it is done.
>     But if we fail to allocate the storage, the stack is used instead.
>     This means we must wait for the called CPU to finish before
>     continuing.
> 
>     Unfortunatly, some callers do no abide by this hint and act as if
>     the non-wait option is mandatory. The MTRR code for instance will
>     deadlock if the smp_call_function is set to wait. This is because
>     the smp_call_function will wait for the other CPUs to finish their
>     called functions, but those functions are waiting on the caller to
>     continue.
> 
>     This patch changes the generic smp_call_function code to use per cpu
>     variables if the allocation of the data fails for a single CPU call. The
>     smp_call_function_many will fall back to the smp_call_function_single
>     if it fails its alloc. The smp_call_function_single is modified
>     to not force the wait state.
> 
>     Since we now are using a single data per cpu we must synchronize the
>     callers to prevent a second caller modifying the data before the
>     first called IPI functions complete. To do so, I added a flag to
>     the call_single_data called CSD_FLAG_LOCK. When the single CPU is
>     called (which can be called when a many call fails an alloc), we
>     set the LOCK bit on this per cpu data. When the caller finishes
>     it clears the LOCK bit.
> 
>     The caller must wait till the LOCK bit is cleared before setting
>     it. When it is cleared, there is no IPI function using it.
> 
> Signed-off-by: Steven Rostedt <srostedt@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Acked-by: Jens Axboe <jens.axboe@xxxxxxxxxx>
> ---
>  kernel/smp.c |   36 +++++++++++++++++++++++++++++++++---
>  1 files changed, 33 insertions(+), 3 deletions(-)

ok, i've updated the commit in tip/core/urgent with the commit line 
removal and the updated description. Find it below - any objections?

	Ingo

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