From: Kirill Smelkov <firstname.lastname@example.org> To: Linus Torvalds <email@example.com> Cc: Rasmus Villemoes <firstname.lastname@example.org>, Al Viro <email@example.com>, Arnd Bergmann <firstname.lastname@example.org>, Christoph Hellwig <email@example.com>, Greg Kroah-Hartman <firstname.lastname@example.org>, linux-fsdevel <email@example.com>, Linux List Kernel Mailing <firstname.lastname@example.org> Subject: Re: [PATCH 1/3] fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock Date: Sat, 13 Apr 2019 16:54:40 +0000 Message-ID: <20190413165116.GB10314@deco.navytux.spb.ru> (raw) In-Reply-To: <20190412124144.GA22786@deco.navytux.spb.ru> On Fri, Apr 12, 2019 at 03:41:44PM +0300, Kirill Smelkov wrote: > On Thu, Apr 11, 2019 at 09:22:56AM -0700, Linus Torvalds wrote: > > On Thu, Apr 11, 2019 at 5:38 AM Kirill Smelkov <email@example.com> wrote: > > > > > > However file->f_pos writing is still there and it will bug under race > > > detector, e.g. under KTSAN because read and write can be running > > > simultaneously. Maybe it is not only race bug, but also a bit of > > > slowdown as read and write code paths write to the same memory thus > > > needing inter-cpu synchronization if read and write handlers are on > > > different cpus. However for this I'm not sure. > > > > I doubt it's noticeable, but it would probably be good to simply not > > do the write for streams. > > > > That *could* be done somewhat similarly, with just changing th address > > to be updated, or course. > > > > In fact, we *used* to (long ago) pass in the address of "file->f_pos" > > itself to the low-level read/write routines. We then changed it to do > > that indirection through a local copy of pos (and > > file_pos_read/file_pos_write) because we didn't do the proper locking, > > so different read/write versions could mess with each other (and with > > lseek). > > > > But one of the things that commit 9c225f2655e36 ("vfs: atomic f_pos > > accesses as per POSIX") did was to add the proper locking at least for > > the cases that we care about deeply, so we *could* say that we have > > three cases: > > > > - FMODE_ATOMIC_POS: properly locked, > > - FMODE_STREAM: no pos at all > > - otherwise a "mostly don't care - don't mix!" > > > > and so we could go back to not copying the pos at all, and instead do > > something like > > > > loff_t *ppos = f.file->f_mode & FMODE_STREAM ? NULL : &file->f_pos; > > ret = vfs_write(f.file, buf, count, ppos); > > > > and perhaps have a long-term plan to try to get rid of the "don't mix" > > case entirely (ie "if you use f_pos, then we'll do the proper > > locking") > > > > (The above is obviously surrounded by the fdget_pos()/fdput_pos() that > > implements the locking decision). > > Ok Linus, thanks for feedback. Please consider applying or queueing the > following patches. Resending those patches one by one in separate emails, if maybe 2 patches inline was problematic to handle...
next prev parent reply index Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-03-26 22:20 Kirill Smelkov 2019-03-26 23:22 ` [PATCH 3/3] fuse: Add FOPEN_STREAM and use stream_open() if filesystem returned that from open handler Kirill Smelkov 2019-04-24 7:13 ` [RESEND, PATCH " Kirill Smelkov [not found] ` <20190424160611.2A71321900@mail.kernel.org> 2019-04-24 19:16 ` Kirill Smelkov [not found] ` <firstname.lastname@example.org> 2019-03-27 6:54 ` [PATCH 2/3] *: convert stream-like files from nonseekable_open -> stream_open Lubomir Rintel 2019-03-27 16:58 ` [PATCH 1/3] fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock Juergen Gross 2019-04-06 17:07 ` Linus Torvalds 2019-04-07 20:04 ` Kirill Smelkov 2019-04-08 0:09 ` Linus Torvalds 2019-04-14 7:11 ` Kirill Smelkov [not found] ` <email@example.com> [not found] ` <20190409203807.GA13855@deco.navytux.spb.ru> [not found] ` <firstname.lastname@example.org> 2019-04-11 12:38 ` Kirill Smelkov 2019-04-11 16:22 ` Linus Torvalds 2019-04-12 12:42 ` Kirill Smelkov 2019-04-13 16:54 ` Kirill Smelkov [this message] 2019-04-13 16:54 ` [PATCH 1/2] vfs: pass ppos=NULL to .read()/.write() of FMODE_STREAM files Kirill Smelkov 2019-04-13 17:27 ` Linus Torvalds 2019-04-13 17:38 ` Al Viro 2019-04-13 18:44 ` Kirill Smelkov 2019-04-13 16:55 ` [PATCH 2/2] vfs: use &file->f_pos directly on files that have position Kirill Smelkov
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