netdev - Re: [PATCH v2 0/9] Remove spin_unlock

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Date:   Fri, 7 Jul 2017 10:44:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        David Laight <David.Laight@...LAB.COM>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "oleg@...hat.com" <oleg@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "dave@...olabs.net" <dave@...olabs.net>,
        "manfred@...orfullife.com" <manfred@...orfullife.com>,
        "tj@...nel.org" <tj@...nel.org>, "arnd@...db.de" <arnd@...db.de>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "will.deacon@....com" <will.deacon@....com>,
        "stern@...land.harvard.edu" <stern@...land.harvard.edu>,
        "parri.andrea@...il.com" <parri.andrea@...il.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 0/9] Remove spin_unlock_wait()

On Fri, Jul 07, 2017 at 10:31:28AM +0200, Ingo Molnar wrote:
> Here's a quick list of all the use cases:
> 
>  net/netfilter/nf_conntrack_core.c:
> 
>    - This is I believe the 'original', historic spin_unlock_wait() usecase that
>      still exists in the kernel. spin_unlock_wait() is only used in a rare case, 
>      when the netfilter hash is resized via nf_conntrack_hash_resize() - which is 
>      a very heavy operation to begin with. It will no doubt get slower with the 
>      proposed changes, but it probably does not matter. A networking person 
>      Acked-by would be nice though.
> 
>  drivers/ata/libata-eh.c:
> 
>    - Locking of the ATA port in ata_scsi_cmd_error_handler(), presumably this can
>      race with IRQs and ioctls() on other CPUs. Very likely not performance 
>      sensitive in any fashion, on IO errors things stop for many seconds anyway.
> 
>  ipc/sem.c:
> 
>    - A rare race condition branch in the SysV IPC semaphore freeing code in 
>      exit_sem() - where even the main code flow is not performance sensitive, 
>      because typical database workloads get their semaphore arrays during startup 
>      and don't ever do heavy runtime allocation/freeing of them.
> 
>  kernel/sched/completion.c:
> 
>    - completion_done(). This is actually a (comparatively) rarely used completion 
>      API call - almost all the upstream usecases are in drivers, plus two in 
>      filesystems - neither usecase seems in a performance critical hot path. 
>      Completions typically involve scheduling and context switching, so in the 
>      worst case the proposed change adds overhead to a scheduling slow path.
> 

You missed the one in do_exit(), which I thought was the original one.