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    SubjectRe: [PATCH] [3/11] SYSCTL: Add proc_rcu_string to manage sysctls using rcu strings
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    "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.

    Eric



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