|From:||Jamie Lokier <jamie-AT-shareable.org>|
|To:||Christoph Hellwig <hch-AT-infradead.org>|
|Subject:||Re: adding proper O_SYNC/O_DSYNC, was Re: O_DIRECT and barriers|
|Date:||Fri, 28 Aug 2009 17:44:32 +0100|
|Cc:||Ulrich Drepper <drepper-AT-redhat.com>, linux-fsdevel-AT-vger.kernel.org, linux-kernel-AT-vger.kernel.org|
Christoph Hellwig wrote: > On Thu, Aug 27, 2009 at 10:24:28AM -0700, Ulrich Drepper wrote: > > The problem with O_* extensions is that the syscall doesn't fail if the > > flag is not handled. This is a problem in the open implementation which > > can only be fixed with a new syscall. > > > > Why cannot just go on and say we interpret O_SYNC like O_SYNC and > > O_SYNC|O_DSYNC like O_DSYNC. The POSIX spec explicitly requires that > > the latter handled like O_SYNC. > > > > We could handle it by allocating two bits, only one is handled in the > > kernel. If the O_DSYNC definition for userlevel would be different from > > the kernel definition then the kernel could interpret O_SYNC|O_DSYNC > > like O_DSYNC. The libc would then have to translate the userlevel > > O_DSYNC into the kernel O_DSYNC. If the libc is too old for the kernel > > and the application, the userlevel flag would be passed to the kernel > > and nothing bad happens. > > What about hte following variant: > > - given that our current O_SYNC really is and always has been actuall > Posix O_DSYNC keep the numerical value and rename it to O_DSYNC in > the headers. > - Add a new O_SYNC definition: > > #define O_SYNC (O_DSYNC|O_REALLY_SYNC) > > and do full O_SYNC handling in new kernels if O_REALLY_SYNC is > present. That looks good for the kernel. However, for userspace, there's an issue with applications which were compiled with an old libc and used O_SYNC. Most of them probably expected O_SYNC behaviour but all they got was O_DSYNC, because Linux didn't do it right. But they *didn't know* that. When using a newer kernel which actually implements O_SYNC behaviour, I'm thinking those applications which asked for O_SYNC should get it, even though they're still linked with an old libc. That's because this thread is the first time I've heard that Linux O_SYNC was really the weaker O_DSYNC in disguise, and judging from the many Googlings I've done about O_SYNC in applications and on different OS, it'll be news to other people too. (I always thought the "#define O_DSYNC O_SYNC" was because Linux didn't implement the weaker O_DSYNC). (Oh, and Ulrich: Why is there a "#define O_RSYNC O_SYNC" in the Glibc headers? That doesn't make sense: O_RSYNC has nothing to do with writing.) To achieve that, libc could implement two versions of open() at the same time as it updates header files. The new libc's __old_open() would do: /* Only O_DSYNC is set for apps built against old libc which were compiled if (flags & O_DSYNC) flags |= O_SYNC; I'm not exactly sure how symbol versioning works, but perhaps the header file in the new libc would need __REDIRECT_NTH to map open() to __new_open(), which just calls the kernel. This is to ensure .o and .a files built with an old libc's headers but then linked to a new libc will get __old_open(). Although libc's __new_open() could have this: /* Old kernels only look at O_DSYNC. It's better than nothing. */ if (flags & O_SYNC) flags |= O_DSYNC; Imho, it's better to not do that, and instead have #define O_SYNC (O_DSYNC|__O_SYNC_KERNEL) as Chris suggests, in the libc header the same as the kernel header, because that way applications which use the syscall() function or have to invoke a syscall directly (I've seen clone-using code doing it), won't spontaneously start losing their O_SYNCness on older kernels. Unless there is some reason why "flags &= ~O_SYNC" is not permitted to clear the O_DSYNC flag, or other reason why they must be separate flags. -- Jamie -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to email@example.com More majordomo info at http://vger.kernel.org/majordomo-info.html
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