• KOSAKI Motohiro's avatar
    futexes: Remove rw parameter from get_futex_key() · 7485d0d3
    KOSAKI Motohiro authored
    Currently, futexes have two problem:
    
    A) The current futex code doesn't handle private file mappings properly.
    
    get_futex_key() uses PageAnon() to distinguish file and
    anon, which can cause the following bad scenario:
    
      1) thread-A call futex(private-mapping, FUTEX_WAIT), it
         sleeps on file mapping object.
      2) thread-B writes a variable and it makes it cow.
      3) thread-B calls futex(private-mapping, FUTEX_WAKE), it
         wakes up blocked thread on the anonymous page. (but it's nothing)
    
    B) Current futex code doesn't handle zero page properly.
    
    Read mode get_user_pages() can return zero page, but current
    futex code doesn't handle it at all. Then, zero page makes
    infinite loop internally.
    
    The solution is to use write mode get_user_page() always for
    page lookup. It prevents the lookup of both file page of private
    mappings and zero page.
    
    Performance concerns:
    
    Probaly very little, because glibc always initialize variables
    for futex before to call futex(). It means glibc users never see
    the overhead of this patch.
    
    Compatibility concerns:
    
    This patch has few compatibility issues. After this patch,
    FUTEX_WAIT require writable access to futex variables (read-only
    mappings makes EFAULT). But practically it's not a problem,
    glibc always initalizes variables for futexes explicitly - nobody
    uses read-only mappings.
    Reported-by: default avatarHugh Dickins <hugh.dickins@tiscali.co.uk>
    Signed-off-by: default avatarKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Acked-by: default avatarPeter Zijlstra <peterz@infradead.org>
    Acked-by: default avatarDarren Hart <dvhltc@us.ibm.com>
    Cc: <stable@kernel.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Cc: Nick Piggin <npiggin@suse.de>
    Cc: Ulrich Drepper <drepper@gmail.com>
    LKML-Reference: <20100105162633.45A2.A69D9226@jp.fujitsu.com>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
    7485d0d3
futex.c 67.7 KB