[lkml]   [2018]   [Jun]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH] infiniband: fix a subtle race condition
On Thu, Jun 14, 2018 at 12:01 AM, Leon Romanovsky <> wrote:
> On Wed, Jun 13, 2018 at 11:21:54PM -0700, Cong Wang wrote:
>> On Wed, Jun 13, 2018 at 10:34 PM, Leon Romanovsky <> wrote:
>> >
>> > 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.
>> Nope, check the assembly if you don't trust me, at least
>> my compiler always fetches ctx->file without this patch.
>> I can show you the assembly code tomorrow (too late to
>> access my dev machine now).
> I trust you, so don't need to check it however wanted to emphasize
> that your solution is compiler specific and not universally true.

So are you saying even with my patch compiler could still re-fetch
ctx->file? I doubt...

>> >
>> > You need to ensure that ucma_modify_id() doesn't run in parallel to
>> > anything that uses "ctx->file" directly and indirectly.
>> >
>> Talk is easy, show me the code. :) I knew there is probably
>> some other race with this code even after my patch, possibly with
>> ->close() for example, but for this specific unlock warning, this patch
>> is sufficient. I can't solve all the races in one patch.
> We do prefer complete solution once the problem is fully understood.

The unlock imbalance problem is fully understood and is clearly shown
in my changelog.

My patch never intends to solve any other problem except this one.

> It looks like you are one step away from final patch. It will be conversion
> of mutex to be rwlock and separating between read (almost in all places)
> and write (in ucma_migrate_id) paths.

Excuse me. How does this even solve the imbalance problem?

f = ctx->file;
ucma_lock_files(f, new_file); // write sem
ctx->file = new_file
ucma_lock_files(f, new_file); // write sem
down_read(&f->rw); // still the old file, nothing change
f = ctx->file; // new file
up_read(&f->rw); // still imbalance

 \ /
  Last update: 2018-06-14 18:58    [W:0.048 / U:2.136 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site