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    SubjectRe: [PATCH] [3/11] SYSCTL: Add proc_rcu_string to manage sysctls using rcu strings
    On Mon, Dec 21, 2009 at 07:00:44PM -0800, Eric W. Biederman wrote:
    > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> writes:
    >
    > > On Mon, Dec 21, 2009 at 02:20:24AM +0100, Andi Kleen wrote:
    > >>
    > >> Add a helper to use the new rcu strings for managing access
    > >> to text sysctls. Conversions will be in follow-on patches.
    > >>
    > >> An alternative would be to use seqlocks here, but RCU seemed
    > >> cleaner.
    > >>
    > >> Signed-off-by: Andi Kleen <ak@linux.intel.com>
    > >
    > > Using the below as an example of my concern about access_rcu_string(), FYI.
    > >
    > >> ---
    > >> include/linux/sysctl.h | 2 +
    > >> kernel/sysctl.c | 66 +++++++++++++++++++++++++++++++++++++++++++++++++
    > >> kernel/sysctl_check.c | 1
    > >> 3 files changed, 69 insertions(+)
    > >>
    > >> Index: linux-2.6.33-rc1-ak/include/linux/sysctl.h
    > >> ===================================================================
    > >> --- linux-2.6.33-rc1-ak.orig/include/linux/sysctl.h
    > >> +++ linux-2.6.33-rc1-ak/include/linux/sysctl.h
    > >> @@ -969,6 +969,8 @@ typedef int proc_handler (struct ctl_tab
    > >>
    > >> extern int proc_dostring(struct ctl_table *, int,
    > >> void __user *, size_t *, loff_t *);
    > >> +extern int proc_rcu_string(struct ctl_table *, int,
    > >> + void __user *, size_t *, loff_t *);
    > >> extern int proc_dointvec(struct ctl_table *, int,
    > >> void __user *, size_t *, loff_t *);
    > >> extern int proc_dointvec_minmax(struct ctl_table *, int,
    > >> Index: linux-2.6.33-rc1-ak/kernel/sysctl.c
    > >> ===================================================================
    > >> --- linux-2.6.33-rc1-ak.orig/kernel/sysctl.c
    > >> +++ linux-2.6.33-rc1-ak/kernel/sysctl.c
    > >> @@ -50,6 +50,7 @@
    > >> #include <linux/ftrace.h>
    > >> #include <linux/slow-work.h>
    > >> #include <linux/perf_event.h>
    > >> +#include <linux/rcustring.h>
    > >>
    > >> #include <asm/uaccess.h>
    > >> #include <asm/processor.h>
    > >> @@ -2016,6 +2017,60 @@ static int _proc_do_string(void* data, i
    > >> }
    > >>
    > >> /**
    > >> + * proc_rcu_string - sysctl string with rcu protection
    > >> + * @table: the sysctl table
    > >> + * @write: %TRUE if this is a write to the sysctl file
    > >> + * @buffer: the user buffer
    > >> + * @lenp: the size of the user buffer
    > >> + * @ppos: file position
    > >> + *
    > >> + * Handle a string sysctl similar to proc_dostring.
    > >> + * The main difference is that the data pointer in the table
    > >> + * points to a pointer to a string. The string should be initially
    > >> + * pointing to a statically allocated (as a C object, not on the heap)
    > >> + * default. When it is replaced old uses will be protected by
    > >> + * RCU. The reader should use rcu_read_lock()/unlock() or
    > >> + * access_rcu_string().
    > >> + */
    > >> +int proc_rcu_string(struct ctl_table *table, int write,
    > >> + void __user *buffer, size_t *lenp, loff_t *ppos)
    > >> +{
    > >> + int ret;
    > >> +
    > >> + if (write) {
    > >> + /* protect writers against each other */
    > >> + static DEFINE_MUTEX(rcu_string_mutex);
    > >> + char *old;
    > >> + char *new;
    > >> +
    > >> + new = alloc_rcu_string(table->maxlen, GFP_KERNEL);
    > >> + if (!new)
    > >> + return -ENOMEM;
    > >> + mutex_lock(&rcu_string_mutex);
    > >> + old = *(char **)(table->data);
    > >> + strcpy(new, old);
    > >> + ret = _proc_do_string(new, table->maxlen, write, buffer, lenp, ppos);
    > >> + rcu_assign_pointer(*(char **)(table->data), new);
    > >> + /*
    > >> + * For the first initialization allow constant strings.
    > >> + */
    > >> + if (!kernel_address((unsigned long)old))
    > >> + free_rcu_string(old);
    > >> + mutex_unlock(&rcu_string_mutex);
    > >> + } else {
    > >> + char *str;
    > >> +
    > >> + str = access_rcu_string(*(char **)(table->data), table->maxlen,
    > >> + GFP_KERNEL);
    > >
    > > So the above statement picks up table->data, then some other CPU comes
    > > in and executes the "write" side of this "if" statement, we get
    > > preempted before access_rcu_string() enters its RCU read-side critical
    > > section, the grace period elapse, we resume, and ... ouch!
    > >
    > > One trick would be to make access_rcu_string() be a macro that does
    > > first access to its first argument in an RCU read-side critical section.
    > > Alternatively, pass in the address of the pointer, rather than the
    > > pointer itself.
    > >
    > > Or explain to me how I am confused.
    >
    > That sounds correct to me. There is also the missing rcu_dereference.
    >
    > Which is less important but it would make clear that access_rcu_string
    > does the dereference outside of the rcu critical section.

    Good point, there does indeed need to be an rcu_dereference() in any case.

    Thanx, Paul


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