Date: Sun, 23 Sep 2012 19:08:47 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Rick Macklem <rmacklem@uoguelph.ca> Cc: FS List <freebsd-fs@freebsd.org> Subject: Re: testing/review of atomic export update patch Message-ID: <20120923160847.GN37286@deviant.kiev.zoral.com.ua> In-Reply-To: <1697573610.1030942.1348267121541.JavaMail.root@erie.cs.uoguelph.ca> References: <20120921080516.GC37286@deviant.kiev.zoral.com.ua> <1697573610.1030942.1348267121541.JavaMail.root@erie.cs.uoguelph.ca>
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--PrbR2RMKSe7tzlp9 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Sep 21, 2012 at 06:38:41PM -0400, Rick Macklem wrote: > Konstantin Belousov wrote: > > On Thu, Sep 20, 2012 at 05:55:26PM -0400, Rick Macklem wrote: > > > Konstantin Belousov wrote: > > > > On Tue, Sep 18, 2012 at 09:34:54AM -0400, Rick Macklem wrote: > > > > > Konstantin Belousov wrote: > > > > > > On Mon, Sep 17, 2012 at 05:32:44PM -0400, Rick Macklem wrote: > > > > > > > Konstantin Belousov wrote: > > > > > > > > On Sun, Sep 16, 2012 at 05:41:25PM -0400, Rick Macklem > > > > > > > > wrote: > > > > > > > > > Hi, > > > > > > > > > > > > > > > > > > There is a simple patch at: > > > > > > > > > http://people.freebsd.org/~rmacklem/atomic-export.patch > > > > > > > > > that can be applied to a kernel + mountd, so that the > > > > > > > > > new > > > > > > > > > nfsd can be suspended by mountd while the exports are > > > > > > > > > being > > > > > > > > > reloaded. It adds a new "-S" flag to mountd to enable > > > > > > > > > this. > > > > > > > > > (This avoids the long standing bug where clients receive > > > > > > > > > ESTALE > > > > > > > > > replies to RPCs while mountd is reloading exports.) > > > > > > > > > > > > > > > > This looks simple, but also somewhat worrisome. What would > > > > > > > > happen > > > > > > > > if the mountd crashes after nfsd suspension is requested, > > > > > > > > but > > > > > > > > before > > > > > > > > resume was performed ? > > > > > > > > > > > > > > > > Might be, mountd should check for suspended nfsd on start > > > > > > > > and > > > > > > > > unsuspend > > > > > > > > it, if some flag is specified ? > > > > > > > Well, I think that happens with the patch as it stands. > > > > > > > > > > > > > > suspend is done if the "-S" option is specified, but that is > > > > > > > a > > > > > > > no op > > > > > > > if it is already suspended. The resume is done no matter > > > > > > > what > > > > > > > flags > > > > > > > are provided, so mountd will always try and do a "resume". > > > > > > > --> get_exportlist() is always called when mountd is started > > > > > > > up > > > > > > > and > > > > > > > it does the resume unconditionally when it completes. > > > > > > > If mountd repeatedly crashes before completing > > > > > > > get_exportlist() > > > > > > > when it is started up, the exports will be all messed > > > > > > > up, so > > > > > > > having the nfsd threads suspended doesn't seem so bad > > > > > > > for > > > > > > > this > > > > > > > case (which hopefully never happens;-). > > > > > > > > > > > > > > Both suspend and resume are just no ops for unpatched > > > > > > > kernels. > > > > > > > > > > > > > > Maybe the comment in front of "resume" should explicitly > > > > > > > explain > > > > > > > this, instead of saying resume is harmless to do under all > > > > > > > conditions? > > > > > > > > > > > > > > Thanks for looking at it, rick > > > > > > I see. > > > > > > > > > > > > My another note is that there is no any protection against > > > > > > parallel > > > > > > instances of suspend/resume happen. For instance, one thread > > > > > > could > > > > > > set > > > > > > suspend_nfsd =3D 1 and be descheduled, while another executes > > > > > > resume > > > > > > code sequence meantime. Then it would see suspend_nfsd !=3D 0, > > > > > > while > > > > > > nfsv4rootfs_lock not held, and tries to unlock it. It seems > > > > > > that > > > > > > nfsv4_unlock would silently exit. The suspending thread > > > > > > resumes, > > > > > > and obtains the lock. You end up with suspend_nfsd =3D=3D 0 but > > > > > > lock > > > > > > held. > > > > > Yes. I had assumed that mountd would be the only thing using > > > > > these > > > > > syscalls > > > > > and it is single threaded. (The syscalls can only be done by > > > > > root > > > > > for the > > > > > obvious reasons.;-) > > > > > > > > > > Maybe the following untested version of the syscalls would be > > > > > better, since > > > > > they would allow multiple concurrent calls to either suspend or > > > > > resume. > > > > > (There would still be an indeterminate case if one thread called > > > > > resume > > > > > concurrently with another few calling suspend, but that is > > > > > unavoidable, > > > > > I think?) > > > > > > > > > > Again, thanks for the comments, rick > > > > > --- untested version of syscalls --- > > > > > } else if ((uap->flag & NFSSVC_SUSPENDNFSD) !=3D 0) { > > > > > NFSLOCKV4ROOTMUTEX(); > > > > > if (suspend_nfsd =3D=3D 0) { > > > > > /* Lock out all nfsd threads */ > > > > > igotlock =3D 0; > > > > > while (igotlock =3D=3D 0 && suspend_nfsd =3D=3D 0) { > > > > > igotlock =3D nfsv4_lock(&nfsv4rootfs_lock, 1, > > > > > NULL, NFSV4ROOTLOCKMUTEXPTR, NULL); > > > > > } > > > > > suspend_nfsd =3D 1; > > > > > } > > > > > NFSUNLOCKV4ROOTMUTEX(); > > > > > error =3D 0; > > > > > } else if ((uap->flag & NFSSVC_RESUMENFSD) !=3D 0) { > > > > > NFSLOCKV4ROOTMUTEX(); > > > > > if (suspend_nfsd !=3D 0) { > > > > > nfsv4_unlock(&nfsv4rootfs_lock, 0); > > > > > suspend_nfsd =3D 0; > > > > > } > > > > > NFSUNLOCKV4ROOTMUTEX(); > > > > > error =3D 0; > > > > > } > > > > > > > > From the cursory look, this variant is an improvement, mostly by > > > > taking > > > > the interlock before testing suspend_nfsd, and using the while > > > > loop. > > > > > > > > Is it possible to also make the sleep for the lock interruptible ? > > > > So that blocked mountd could be killed by a signal ? > > > Well, it would require some coding. An extra argument to > > > nfsv4_lock() > > > to indicate to do so and then either the caller would have to check > > > for a pending termination signal when it returns 0 (indicates didn't > > > get > > > lock) or a new return value to indicate EINTR. The latter would > > > require > > > all the calls to it to be changed to recognize the new 3rd return > > > case. > > > Because there are a lot of these calls, I'd tend towards just having > > > the > > > caller check for a pending signal. > > > > > > Not sure if it would make much difference though. The only time it > > > would get stuck in nfsv4_lock() is if the nfsd threads are all > > > wedged > > > and in that case having mountd wedged too probably doesn't make much > > > difference, since the NFS service is toast in that case anyhow. > > > > > > If you think it is worth doing, I can add that. I basically see this > > > as a "stop-gap" fix until such time as something like nfse is done, > > > but since I haven't the time to look at nfse right now, I have no > > > idea when/if that might happen. > >=20 > > Ok, please go ahead with the patch. Having the patch even in its > > current > > form is obviously better then not to have it. If the wedged mountd > > appears to be annoying enough for me, I would do the change. > >=20 > > Thanks. > Oops, I spoke too soon. When I took a look at the code, I realized that > having nfsv4_lock() return when a pending signal interrupts the msleep() > isn't easy. As such, I think I'll leave it out of the patch for now. >=20 > For those who find these things interesting, the reason the above is > hard is the funny intentional semantics that nfsv4_lock() implements. > Most of the time, the nfsd threads can concurrently handle the NFSv4 > state structures, using a mutex to serialize access to the lists and > never sleeping while doing so. However, there are a few case (mainly > delegation recall) where sleeping and knowing that no other thread > is modifying the lists is necessary. >=20 > As such, nfsv4_lock() will be called by potentially many nfsd threads > (up to 200+) wanting this exclusive sleep lock. However, it is coded > so that only the first one that wakes up after the shared locks (I call > it a ref count in the code) have been released, gets the exclusive > lock. The rest of the threads wake up after the first thread releases > the exclusive lock, but simply return without getting the lock. > This avoids up to 200+ threads getting the exclusive lock in turn and > then going "oh, I don't need it since that other thread already got > the work done" so it releases the exclusive lock and gets a shared one. > (It also implements the exclusive lock as having priority over the shared > lock request, so that nfsv4_lock() won't wait indefinitely for the > exclusive lock.) >=20 > The above is done by having the first thread that wakes up once the > shared locks are released clear the "want an exclusive lock" flag > as it acquires it. >=20 > If a call were to return due to a signal, it wouldn't know if it > should clear the "want an exclusive lock" flag or not, since it > wouldn't know if other threads currently want it or not. My take is that the thread which got EINTR from msleep() shall not clear the flag. It shall not get the lock anyway. >=20 > This could probably be fixed by adding a count of how many threads > are currently sleeping, waiting for the "want an exclusive flag", > but that's too scary for me to do, unless it really is needed. > (As you might have guessed, it's pretty easy to break this in > subtle ways and I'm a chicken;-) BTW, nfsv4_lock/unlock desperately need assertion that the mutex is locked on entry. --PrbR2RMKSe7tzlp9 Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAlBfNA8ACgkQC3+MBN1Mb4jt7ACfQF8T8mGYITcgjY8xZ3AMa4IC baIAoI4aUZ+bsFElAdjwcjhvfey2cLPW =u6Ci -----END PGP SIGNATURE----- --PrbR2RMKSe7tzlp9--
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