Re: [PATCH] smp: prevent soft lockup in smp_call_function_many_cond

[Rik van Riel <riel@surriel.com>]

On Wed, 2026-05-27 at 15:16 +1200, Mark Tomlinson wrote:
> Using the PowerPC P2040 (e500mc) CPU, soft lockups can occasionally
> be
> seen in smp_call_function_many_cond(). The conclusion is that this
> CPU
> does not process the doorbell interrupt while in a data-storage (MMU)
> exception. If more than one CPU in a multi core environment is
> calling
> this function at the same time, it is possible for a deadlock to
> occur.

Does that mean if the CPU in question does not call
smp_call_function_many_cond() while in a data-storage
exception, the system might still hang?

Not that there's anything wrong with reducing the
frequency of what is (presumably) an already pretty
rare hang.

> 
> The fix for this is to call flush_smp_call_function_queue() before
> waiting for responses from other CPUs. If there is something in the
> queue, this is a good time to process it before busy-waiting on other
> CPUs. On other architectures this call will quickly do nothing, as
> the
> queue will be empty.

Agreed, this does look completely harmless at worst,
and does look like it would at the very least improve
that e500mc issue.

> 
> Signed-off-by: Mark Tomlinson <mark.tomlinson@alliedtelesis.co.nz>
> 
Reviewed-by: Rik van Riel <riel@surriel.com>


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