[LU-8249] Potential deadlock in lnet

• Type: Bug
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.9.0
• Labels: patch
• Severity: 3
• Assignee: Doug Oucharek (Inactive)
• Reporter: CEA

Description

While testing a patch I was able to deadlock all the CPU of my machine.
After investigating the log reports I found out that in "LNetMDAttach" (implemented in lnet/lnet/lib-md.c) there is a call to the "vfree" function in interrupt context which is illegal in linux kernel versions prior to 3.10.

I cannot be sure whether or not this was actually the cause of my problem though, because I have not been able to reproduce since then.

Comments

[Gerrit Updater]

Oleg Drokin ([e-mail]) merged in patch http://review.whamcloud.com/20676/
Subject: LU-8249 lnet: potential deadlock in lnet
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: cc025e667464672edd25da819e106854c220e668

[Bruno Faccini (Inactive)]

Sorry, I meant for failures before lnet_res_lock_current(), but I see that this what you also implemented in the new patch-set #4 !

[Quentin Bouget (Inactive)]

Indeed I missed the one in LnetMDBind().
I'm not sure lnet_res_lock_current() can actually fail though, the return code that is tested after it is set by lnet_md_build() and I don't think there is any actual reason to take the spinlock upon failure (if it is to release it right after that).

If this ever occurs again I will make sure to get a crash-dump.

(New patch is available)

[Bruno Faccini (Inactive)]

Hello Quentin, even if I don't think it will fix the specific dead-lock situation you have encountered, I am not against your patch that sounds ok to comply to the rule against calling vfree() with a spin-lock currently granted, but then I think you should at least modify LNetMDBind() the same way and there also not to try to lnet_res_unlock() upon lnet_res_lock_current() failure.

Also, you may have triggered an issue similar to the one tracked in LU-8334, in fact we will need a crash-dump taken upon next occurrence to be able to determine the real root cause.

What do you think?

[Quentin Bouget (Inactive)]

Hello Bruno, I actually do not have much, I did not realize what this was at the time. I thought it was related to my patch and would be easy to reproduce. The best I have is a partial calling stack incriminating "cfs_percpt_lock" (attachment file). I reviewed the code that used it and found the inverted "vfree" and "cfs_percpt_unlock" in "LNetMDAttach".

[Joseph Gmitter (Inactive)]

Hi Doug,

Can you please investigate this patch?

Thanks.
Joe

[Bruno Faccini (Inactive)]

Hello Quentin, could you provide additional infos to help us to understand the problem ?? Like the log reports you refer too, and may be a crash-dump is available too ??

[Gerrit Updater]

Quentin Bouget ([e-mail]) uploaded a new patch: http://review.whamcloud.com/20676
Subject: LU-8249 lnet: potential deadlock in lnet
Project: fs/lustre-release
Branch: master
Current Patch Set: 1
Commit: ef1052386265b73c129c2dc5d20bd86e175a8c0d