From: Shan Hai Date: Thu, 8 Nov 2012 15:57:49 +0000 (+0000) Subject: powerpc/vdso: Remove redundant locking in update_vsyscall_tz() X-Git-Tag: v3.2.37~32 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=615df262c1ab4fbec7e198c5c956a7bca6ef7f22;p=pandora-kernel.git powerpc/vdso: Remove redundant locking in update_vsyscall_tz() commit ce73ec6db47af84d1466402781ae0872a9e7873c upstream. The locking in update_vsyscall_tz() is not only unnecessary because the vdso code copies the data unproteced in __kernel_gettimeofday() but also introduces a hard to reproduce race condition between update_vsyscall() and update_vsyscall_tz(), which causes user space process to loop forever in vdso code. The following patch removes the locking from update_vsyscall_tz(). Locking is not only unnecessary because the vdso code copies the data unprotected in __kernel_gettimeofday() but also erroneous because updating the tb_update_count is not atomic and introduces a hard to reproduce race condition between update_vsyscall() and update_vsyscall_tz(), which further causes user space process to loop forever in vdso code. The below scenario describes the race condition, x==0 Boot CPU other CPU proc_P: x==0 timer interrupt update_vsyscall x==1 x++;sync settimeofday update_vsyscall_tz x==2 x++;sync x==3 sync;x++ sync;x++ proc_P: x==3 (loops until x becomes even) Because the ++ operator would be implemented as three instructions and not atomic on powerpc. A similar change was made for x86 in commit 6c260d58634 ("x86: vdso: Remove bogus locking in update_vsyscall_tz") Signed-off-by: Shan Hai Signed-off-by: Benjamin Herrenschmidt Signed-off-by: Ben Hutchings --- Reading git-diff-tree failed