On Sun, 9 Feb 2014 14:56:15 +0400 Vladimir Davydov <vdavy...@parallels.com> wrote:
> Currently kobject_uevent has somewhat unpredictable semantics. The point > is, since it may call a usermode helper and wait for it to execute > (UMH_WAIT_EXEC), it is impossible to say for sure what lock dependencies > it will introduce for the caller - strictly speaking it depends on what > fs the binary is located on and the set of locks fork may take. There > are quite a few kobject_uevent's users that do not take this into > account and call it with various mutexes taken, e.g. rtnl_mutex, > net_mutex, which might potentially lead to a deadlock. > > Since there is actually no reason to wait for the usermode helper to > execute there, let's make kobject_uevent start the helper asynchronously > with the aid of the UMH_NO_WAIT flag. > > Personally, I'm interested in this, because I really want kobject_uevent > to be called under the slab_mutex in the slub implementation as it used > to be some time ago, because it greatly simplifies synchronization and > automatically fixes a kmemcg-related race. However, there was a deadlock > detected on an attempt to call kobject_uevent under the slab_mutex (see > https://lkml.org/lkml/2012/1/14/45), which was reported to be fixed by > releasing the slab_mutex for kobject_uevent. Unfortunately, there was no > information about who exactly blocked on the slab_mutex causing the > usermode helper to stall, neither have I managed to find this out or > reproduce the issue. > > BTW, this is not the first attempt to make kobject_uevent use > UMH_NO_WAIT. Previous one was made by commit f520360d93c, but it was > wrong (it passed arguments allocated on stack to async thread) so it was > reverted (commit 05f54c13cd0c). It targeted on speeding up the boot > process though. The patches look good to me. One is kobject (Greg) and the other is slub (Pekka), so I grabbed them ;) Reviews-and-acks, please? btw, when referring to commits, please use the form f520360d93c ("kobject: don't block for each kobject_uevent") because the same commit can have different hashes in different trees. (Although I suspect the amount of convenience this provides others doesn't match the amount of time I spend fixing changelogs!) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/