From 97df75cde57f0a24075200e22d9e2cfb1f2e195b Mon Sep 17 00:00:00 2001 From: "Joel Fernandes (Google)" Date: Thu, 1 Aug 2019 17:39:16 -0400 Subject: Revert docs from "rcu: Restore barrier() to rcu_read_lock() and rcu_read_unlock()" This reverts docs from commit d6b9cd7dc8e041ee83cb1362fce59a3cdb1f2709. Signed-off-by: Joel Fernandes (Google) [ paulmck: Added Joel's SoB per Stephen Rothwell feedback. ] [ paulmck: Joel approved via private email. ] Signed-off-by: Paul E. McKenney --- .../RCU/Design/Requirements/Requirements.html | 71 ---------------------- 1 file changed, 71 deletions(-) (limited to 'Documentation/RCU') diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index 467251f7fef6..bdbc84f1b949 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -2129,8 +2129,6 @@ Some of the relevant points of interest are as follows:
  • Hotplug CPU.
  • Scheduler and RCU.
  • Tracing and RCU. -
  • -Accesses to User Memory and RCU.
  • Energy Efficiency.
  • Scheduling-Clock Interrupts and RCU. @@ -2523,75 +2521,6 @@ cannot be used. The tracing folks both located the requirement and provided the needed fix, so this surprise requirement was relatively painless. -

    -Accesses to User Memory and RCU

    - -

    -The kernel needs to access user-space memory, for example, to access -data referenced by system-call parameters. -The get_user() macro does this job. - -

    -However, user-space memory might well be paged out, which means -that get_user() might well page-fault and thus block while -waiting for the resulting I/O to complete. -It would be a very bad thing for the compiler to reorder -a get_user() invocation into an RCU read-side critical -section. -For example, suppose that the source code looked like this: - -

    -
    - 1 rcu_read_lock();
- 2 p = rcu_dereference(gp);
- 3 v = p->value;
- 4 rcu_read_unlock();
- 5 get_user(user_v, user_p);
- 6 do_something_with(v, user_v);
-
    -
    - -

    -The compiler must not be permitted to transform this source code into -the following: - -

    -
    - 1 rcu_read_lock();
- 2 p = rcu_dereference(gp);
- 3 get_user(user_v, user_p); // BUG: POSSIBLE PAGE FAULT!!!
- 4 v = p->value;
- 5 rcu_read_unlock();
- 6 do_something_with(v, user_v);
-
    -
    - -

    -If the compiler did make this transformation in a -CONFIG_PREEMPT=n kernel build, and if get_user() did -page fault, the result would be a quiescent state in the middle -of an RCU read-side critical section. -This misplaced quiescent state could result in line 4 being -a use-after-free access, which could be bad for your kernel's -actuarial statistics. -Similar examples can be constructed with the call to get_user() -preceding the rcu_read_lock(). - -

    -Unfortunately, get_user() doesn't have any particular -ordering properties, and in some architectures the underlying asm -isn't even marked volatile. -And even if it was marked volatile, the above access to -p->value is not volatile, so the compiler would not have any -reason to keep those two accesses in order. - -

    -Therefore, the Linux-kernel definitions of rcu_read_lock() -and rcu_read_unlock() must act as compiler barriers, -at least for outermost instances of rcu_read_lock() and -rcu_read_unlock() within a nested set of RCU read-side critical -sections. -

    Energy Efficiency

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