Re: [PATCH] infiniband: fix a subtle race condition

[Leon Romanovsky <leon@kernel.org>]

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On Wed, Jun 13, 2018 at 04:49:47PM -0700, Cong Wang wrote:
> In ucma_event_handler() we lock the mutex like this:
>
> mutex_lock(&ctx->file->mut);
> ...
> mutex_unlock(&ctx->file->mut);
>
> which seems correct, but we could translate it into this:
>
> f = ctx->file;
> mutex_lock(&f->mut);
> ...
> f = ctx->file;
> mutex_unlock(&f->mut);
>
> as the compiler does. And, because ucma_event_handler() is
> called in a workqueue so it could race with ucma_migrate_id(),
> so the following race condition could happen:
>
> CPU0			CPU1
> f = ctx->file;
> 			ucma_lock_files(f, new_file);
> 			ctx->file = new_file
> 			ucma_lock_files(f, new_file);
> mutex_lock(&f->mut); // still the old file!
> ...
> f = ctx->file; // now the new one!!
> mutex_unlock(&f->mut); // unlock new file!
>
> Fix this by reading ctx->file once before mutex_lock(), so we
> won't unlock a different mutex any more.

Hi Cong,

If the compiler optimizes the first line (mutex_lock) as you wrote,
it will reuse "f" for the second line (mutex_unlock) too.

You need to ensure that ucma_modify_id() doesn't run in parallel to
anything that uses "ctx->file" directly and indirectly.

Thanks

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